The present invention relates to a storage system, NAS (Network Attached Storage) server, and snapshot acquisition method, more particularly to a technique for applying a global namespace formed by grouping namespaces of one or more local file systems to snapshots of the respective file systems.
NAS, in which a storage system is connected to a network and managed as a shared disk for computers connected to the network well known. The NAS includes a NAS server having a network interface and a disk drive (or a disk array system combining a number of disk drives) for storing data. In recent years, a plurality of NAS servers has been combined to provide one service to deal with increase in storage capacity and sharing of load. In the above described NAS server management, a global namespace is well known, where namespaces of file systems managed by each NAS server are grouped into one namespace to eliminate the need for changes in the settings in the client required as a result of changes in the configuration of the file systems managed by each NAS server.
For example, U.S. Pat. No. 6,671,773 discloses a technique for presenting file systems including a plurality of network elements and disk elements as a single file system.
Meanwhile, a technique called “snapshot” for taking, at a certain time, the entire image of a file system managed by the NAS for the purpose of backing up or recovering a file or file system is well known. For example, a snapshot of a file system is obtained regularly and mounted to a predetermined local directory. Even if a failure occurs in a file or file system, the broken file or file system can be recovered by using the snapshot taken when that file or file system was working properly. Moreover, a user can perform recovery by him/herself, without an administrator. It is an advantage for the user. Usually, a snapshot is not a copy of data itself, but a technique for copying data link information and maintaining only differential data in updated data. Accordingly, a snapshot can be taken within a comparatively short time. The LVM (Logical Volume Manager) in Linux (Registered Trademark) has a snapshot function.